The present invention relates to a magneto-optical recording medium for a magneto-optical memory, and/or a magnetic recording display element, in particular, relates to such a medium which is a magnetic thin film having an easy magnetization axis in the depth direction of a film, which stores information by producing reversed magnetic domain in a circular or arbitrary shape, and reading stored information by using magneto-optical effect like Kerr effect.
Ferro-magnetic thin film with an easy magnetization axis in the depth direction (which is perpendicular to a film surface) can have a small reversed magnetic domain in a uniformly magnetized film. Digital information is stored in that film when the presence and non-presence of that reversed magnetic domain are designated to the information "1" and "0", respectively. When that ferro-magnetic film has high coercive force at room temperature, and has a Curie point or a magnetic compensation temperature close to room temperature, that film is used as a beam addressable file in which reversed magnetic domain is provided at a desired location by using an optical beam and the effect of the Curie point or the magnetic compensation temperature.
FIG. 3 shows the principle of the magneto-optical recording which is used in the present invention. In the figure, the numeral 10 is a recording medium which has an easy magnetization axis 18 in the depth direction of the film. The numeral 12 is a lens for focusing the laser beam 14. It is assumed that the film is pre-magnetized uniformly in the predetermined polarity as indicated by the numeral 18, and an external bias magnetic field 16 is applied to the film 10 so that the direction of the external field 16 is opposite to the original magnetic field 18, and the external field 16 is less than the coercive force of the film 10. Since the external field 16 is less than the coercive force, the pre-magnetized field 18 is not reversed by the external field 16. In the above situation, when the film is selectively heated by the laser beam 14, the temperature of the heated portion becomes higher than the Curie point or the magnetic compensation temperature at which the film has a very small coercive force H.sub.c. Therefore, the internal magnetic field at the heated area in the film 10 coincides with the external magnetic field 16. That is to say, the internal magnetic field 18 is reversed to the reversed field 20 by illuminating (or heating) the film selectively. Thus, information is stored in the shape of a reversed magnetic domain by illuminating the film selectively.
The stored information is read by using the Kerr effect, that is to say, the surface of the film 10 is illuminated by a weak laser beam which is subject to rotate the plane of polarization according to the direction of the magnetization of the film 10 on the principle of the Kerr effect. The rotation of the plane of polarization is converted to the beam strength by using a beam analyzer and detector to provide read-out information. That film may be a beam addressable storage means when the beam scans the surface of the flim.
The preferred nature of the magneto-optical recording medium is, therefore, (1) the Curie point or the magnetic compensation temperature is low or close to room temperature so that the recording laser power may be low, and (2) the Kerr effect rotation is large so that high reproducing signal or high signal-to-noise ratio is obtained.
A conventional beam addressable file by a ferromagnetic film with an easy magnetization axis in the depth direction is a polycrystal metal thin film like MnBi, an amorphous metal thin film like Gd-Co, Gd-Fe, Tb-Fe, Dy-Fe, and compound single crystal thin film like GIG.
A polycrystal metal thin film like MnBi which stores information by using the Curie point effect has high coercive force (about 5 kilo Oe at room temperature) which is advantageous as a recording medium, however, it has the disadvantage that it needs high power for storing information since the Curie point is high (the Curie point Tc of MnBi is Tc=360.degree. C.). Further, it is difficult to produce a film with precise ratio of compositions.
An amorphous metal thin film like Gd-Co, Gd-Fe et al which stores information by using magnetic compensation temperature has the advantages that it can be produced on any substrate, and the magnetic compensation temperature can be adjusted by including some impurities. However, it has the disadvantages that the coercive force at room temperature is small (300-500 Oe), and therefore, the stored signal might be unstable. Further, it is difficult to produce a film since the composition ratio must be controlled very accurately (1 atom % or less).
A compound single crystal thin film like GIG has the disadvantage that the producing cost is considerably high.
An amorphous alloy thin film TbFe or DyFe which includes Tb or Dy by 15-30 atom % has been proposed for overcoming the above disadvantages, and has the following advantages.
(1) It has an easy magnetization axis perpendicular to the film surface, and coercive force higher than 5 kilo or 6 kilo Oe at room temperature, therefore, high density recording is possible, and the stored data is stable and reliable.
(2) It has high coercive force, therefore, a desired shape of magnetic domain can be written.
(3) An excellent recording medium with high coercive force is obtained with wide range of composition ratio. Therefore, the composition ratio control is not severe, and the producing process is simple and the producing yield rate can be high.
(4) The Curie point is low (120.degree. C. for TbFe, and 60.degree. C. for DyFe), therefore, a small amount of power is enough to store information by using Curie point principle.
However, an amorphous alloy thin film like TbFe or DyFe has the disadvantage that the S/N (signal to noise ratio) in reading out stored information optically, because of low Curie point. FIG. 1A and FIG. 1B show optical reproduced signal level S (FIG. 1A), and S/N (FIG. 1B), in optically reading out signal in an amorphous alloy thin film, where the horizontal axis shows laser power I.sub.0 (mili-watt) illuminating the film. It is noted in FIGS. 1A and 1B that TbFe and DyFe which are excellent as a recording medium are worse in optical reading than GdFe which is not as good as TbFe and DyFe in optical recording. The inferior reading out characteristics come from a low Curie point, that is to say, when a laser power I.sub.0 in the reproducing phase is too high, the stored information itself is broken by said high reproducing laser beam, and thus, the S/N is deteriorated. That disadvantage of worse reading characteristics of TbFe and DyFe is a key disadvantage as a magneto-optical memory.
Ternary thin films, like GdTbFe, GdDyFe, TbFeCo, and/or DyFeCo have been proposed for solving the above disadvantages. Those ternary thin films have the nature that they have better reading out characteristics than that of binary film like TbFe or DyFe, because of large Kerr rotation angle of ternary films. However, those three elements films have the disadvantage that the Curie point is high and they need high power for recording information.
As mentioned above, a conventional recording medium does not satisfy recording characteristics and reading characteristics simultaneously.